PACINI, METAMORPHISM OF PORTLAND CEMENT 201 



The temperature and j>ressure of the percolating water being constant, 

 the flow is diminished by cementing and clogging, and increased by ero- 

 sion and solution; the quantity of water flowing through the mortar or 

 concrete therefore is a function of the balancing of these processes. . 



Cementing may result from deposition of material originally in solu- 

 tion in the percolating water, or dissolved from one portion of the 

 structure and deposited in another. 



Clogging, similarly, results from material originally in suspension in 

 the percolating water, and deposited in the pores of the concrete, or from 

 material eroded from one part of the mass, either mechanically or as a 

 result of solution of the attacking portions, and deposited in another 

 part. 



Erosion per se is a negligible factor; that is, the flow of pure water, 

 carrying no suspended matter, will have very small mechanical effect 

 upon an insoluble material. "When the water is armed with suspended 

 matter, however, its corrasive effects become proportionally magnified. 



Solution is the most important factor in the process of percolation. 

 Following the order laid down by Van Hise for natural rocks (104, p. 

 536), the basic materials removed are, firstly, the alkalies and, secondly, 

 the alkaline earths, in the order calcium, magnesium. Since the alkalies 

 exist in cement in the proportion of a little over one per cent and are 

 not essential to the hydraulic properties or the strength, their solution 

 is a matter of little consequence, except in that it may result in the for- 

 mation of solutions which react upon the lime compounds and render 

 their solution more easy of accomplishment. This reaction has been 

 considered elsewhere. The removal of magnesium compounds proceeds 

 at a lesser rate, although there is a greater percentage of them present; 

 and their removal, in the main, may be dismissed as insignificant. 



Since more than half the weight of fresh cement consists of lime, and 

 since the strength of cement depends for the greater part upon calcium 

 hydroxide, whether crystalline or adsorbed by colloids, the removal of 

 calcium hydroxide from set cement is the factor of the greatest impor- 

 tance. Considering its solubility in pure water, the reversion of the 

 hydroxide to the crystalline form tends to diminish its solubility, or 

 from the other standpoint, its adsorption by a colloid tends to remove it 

 from the solvent action of water. Unfortunately, however, it must be 

 borne in mind that without exception, cement structures are nowhere 

 subject to the action of pure water alone. From rain water, with its 

 appreciable burden of dissolved gases and atmospheric salts, to the 

 water of the ocean and the more heavily laden rock and mine waters, 

 concrete structures are everywhere in contact with saline solutions of 

 varying concentrations. 



